The invention relates to a process for the production of biocompatible structures and a biocompatible microchip. The description, as will become clear from the following text, may also be entitled: Utilization of CARL processes (chemical amplification of resist lines) for bioelectronics: substrate binding via an insulating layer
Bioelectronics is a rapidly developing research area which combines chemistry, biochemistry and physics. Its aim is to effect communication between electronic apparatuses and living cells. The main feature of a bioelectronic component is the immobilization of a biomaterial on a conductive or semiconductive substrate and the conversion of biological functions associated with the biological material into electronic signals. Examples of microelectronic components by means of which biological functions can be influenced and controlled are cardiac pacemakers and inner ear auditory prostheses. The development of such bioelectronic components leads to increasingly complex systems wherein a large number of transmission channels for information transmission between electronic component and the cells to be influenced are required. Thus, for example, retina implants or prostheses for walking/standing are being developed. It is necessary, for that purpose, to develop implants which, with numerous contact points, can both stimulate nerve tissue in time sequence and detect a large number of nerve signals which can be resolved spatially and with respect to time. However, metallic electrodes as used in cardiac pacemakers are unsuitable here since these are recognized as foreign bodies and thus lead to rejection reactions. Attempts have therefore been made to produce the electrical contact between electronic component and biological tissue with the aid of polymers, such as, for example, silicones or polyurethane. For this purpose, the polymers must however be electrically conductive and additionally biocompatible, i.e. the materials must not give rise to any rejection reaction. In order to be able to contact nerve paths in a specific manner, a structuring of these materials or of the substrates used, for example mini-silicon wafers, i.e., silicon platelets having dimensions in the neighborhood of a few millimeters, is necessary. The dimensions of the structures, such as pyramids or holes, produced in or on the substrate are in a range from 10 xcexcm to about 70 xcexcm. The most critical element in bioelectronics is the interface between electronics and biological tissue. In order to produce suitable contact, the procedure adopted to date, for example, is first to etch about 25 xcexcm deep pyramidal indentations into a silicon chip. The indentations are then first partly filled with conductive silicone and a second layer of nonconductive silicone is then applied. The polymers are then crosslinked and the structured flexible layer is then removed from the silicon chip. Finally, contact with the silicon protuberances formed on the surface of the flexible layer is produced by individual connecting lines.
A similar principle can be used to produce rectangular trenches having tiny dimensions from polyurethane, which trenches can act as microcells for the cultivation of nerve cells. In order to be able to connect individual neurons specifically to microsystems, supporting structures, such as, for example, trench-like microstructures, are provided on the surface of the substrate. Furthermore, adhesion promoters which facilitate the growth of cells on the surface of the substrate are applied to the surface. In such structures, seed cells grow into network-like structures, and biohybrid systems in the form of microchips covered with cell growth form. Materials which promote cell growth and support the adhesion of the cells are suitable as adhesion promoters at the interface.
In spite of the numerous activities in the area of bioelectronics, this area is still in an experimental stage, so that considerable progress is necessary, particularly in the region of the interface between electronic component and cells, in order to make this area accessible to medical use in practical applications.
It is accordingly an object of the invention-to provide a method of producing biocompatible structures and a biocompatible microchip, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a process for the production of biocompatible structures that is simple to carry out and that permits the production of a contact matrix having many contact points.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of producing biocompatible structures, which comprises:
providing a substrate;
depositing a chemically amplified photoresist containing a polymer with anchor groups for linking a biocompatible compound on the substrate;
structuring the photoresist for form a structured resist; and
treating the structured resist with a biocompatible compound to coordinate the biocompatible compound to the anchor groups of the polymer.
The method according to the invention employs a technique as used in the lithographic structuring of semiconductor chips. This technique has been very widely developed and it can be used to produce structure dimensions down to the neighborhood of less than 100 nm. As already mentioned above, structures having dimensions in the region-of about 25 xcexcm are required for bioelectronic applications. Structures having these dimensions can therefore be readily produced using the known photoresists and imaging techniques. The polymer used in the photoresist need only have anchor groups which permit subsequent linking of biocompatible substances. Subsequent modification of photoresists is already known from the structuring of semiconductors. In this process, the resist structures produced on a substrate are subsequently expanded by linking of expansion reagents in order to be able in this way to produce structures whose dimensions are below the resolution limit of the optical apparatuses used for exposure. Such resists and amplification processes are described, for example, in the commonly assigned publications European patent EP 395 917 B1 and U.S. Pat. No. 5,234,793 (CARL: chemical amplification of resist lines; CAR: chemically amplified resist).
In principle, all chemically amplified photoresists as well as all known structuring processes can be used for the production of the biocompatible structures. All that is necessary is that groups which permit the linking of a biocompatible compound are still present on the structured resist. Both positive and negative chemically amplified photoresists can be used. In the case of the positive chemically amplified resists, the exposed sections of the photoresist are removed by means of a developing solution in the development step, while the unexposed parts remain as lands on the substrate. This is achieved by virtue of the fact that the exposure liberates a catalyst, which changes the polymer of the photoresist in its chemical nature so that a substantial differentiation between exposed and unexposed parts is achieved. This can be achieved, for example, by eliminating groups on the polymer, with the result that the polarity of the polymer increases substantially so that it becomes soluble in aqueous developers. It is also possible to use negatively structurable resists, wherein the exposed parts remain on the substrate as lands while the unexposed parts are removed by means of an aqueous developer. The chemical differentiation between unexposed and exposed sections is generally carried out by a procedure wherein the exposure liberates a catalyst which, for example, produces crosslinking of the polymer of the photoresist, with the result that it becomes insoluble in aqueous developers. In the development step, the unexposed parts, which generally have compounds of high polarity, are then removed by means of an aqueous developer. It is also possible to use modified processes which are based on the above-mentioned positive and negative chemically amplified photoresist systems.
Such a process is described, for example, in U.S. Pat. No. 4,491,628. There, that layer of a positive photoresist which is applied to a substrate is first exposed, an acid being liberated from a photo acid generator. In the subsequent amplification step, acid-labile groups are eliminated from the polymer in the exposed parts by heating, so that the polymer is now present in a polar form. In contrast to the positive development process described above, development is now not effected with a polar aqueous developer but a nonpolar solvent is used for the development. Consequently, only the unexposed parts wherein the polymer has retained its original nonpolar form are detached from the substrate. Since the polar fractions of the resist wherein polar groups were produced by the exposure, for example carboxyl groups, are insoluble in nonpolar solvents, they remain as lands on the substrate.
A process as described, for example, in the commonly assigned PCT publication WO 01/42860 A1 can also be used for the production of a structured resist. There, the photoresist contains a photo base as well as a thermo acid. In the exposure of the photoresist, a base is liberated in the exposed parts. If the photoresist is then heated, an acid is liberated from the thermo acid generator. In the exposed parts, the acid is neutralized by the previously liberated base and is therefore no longer available as a catalyst. In the unexposed parts, the acid catalyzes the elimination of acid-labile groups from the polymer. In the unexposed parts, the polymer is therefore converted from its nonpolar form into a polar form. In the subsequent development step, the unexposed parts can therefore be selectively detached from the substrate by means of an aqueous alkaline developer, whereas the exposed parts remain as lands on the substrate.
In all these processes, it is essential that groups for binding the biocompatible compound are still available after the structuring of the photoresist.
However, a process as described in the above-mentioned European patent EP 0 395 917 B1 is preferably used for structuring the photoresist. The photoresist used is a positive photoresist to which, after exposure, amplification and development, the biocompatible compound is linked in a further step.